In recent years, insufficient mineral intake has been pointed out. In relation to prevention of the life-style related diseases, which is considered to be caused by mineral deficiency, health maintenance and the like, roles of various kinds of minerals have started to be emphasized. Under such circumstances, mineral-enriched foods have been increasingly found in the market.
For example, calcium is generally contained only in restricted groups of foods, and therefore the intake thereof is apt to be insufficient. In particularly, osteoporosis, which is a bone deterioration disease, has become a major problem in all the countries of the world in recent years. As a method for calcium enrichment in foods, there are known a method of adding water-soluble calcium such as calcium chloride and calcium lactate, and a method of adding water-insoluble calcium such as calcium carbonate and calcium phosphate.
However, when a water-soluble mineral is added, mineral ions are produced in the solution, so that the flavors of the mineral-containing foods are significantly destroyed by salty taste, bitter taste, astringent taste and the like ascribed to the anions. In addition, there have been problems, including a problem that mineral ions may react with proteins to produce coagulated matter, and may have an adverse effect on the organization or physical properties of the food by reacting with an ingredient in the food. Also, when a water-insoluble mineral is added, there have been problems, including a problem that it is difficult to attain homogenization in foods, for example, precipitation and separation occurs in a short period time due to the high specific gravity, or secondary aggregation of powder of the mineral occurs.
Iron is known to be present in a form bound with a blood protein hemoglobin. Under iron-deficient conditions, iron is compensated from stored iron in the tissues. The condition where stored iron is deficient is called latent anemia, which is a worldwide problem from developing countries to advanced countries. This trend is noticeably found particularly in female senior high school students and young adult females, so that there are many women who develop iron deficiency anemia. A major reason for this is thought to be from daily diet. In the case of women, a characteristic reason is that women are under circumstances to easily develop anemia due to iron deficiency resulting from, for example, menstrual bleeding, increase in required iron during pregnancy, insufficient intake due to being on a diet. In order to compensate this iron deficiency, iron-enriched foods have been commercially available, and many commercial products including milk and soft drinks enriched with iron have been commercially available. For example, in soft drinks and the like for the purpose of iron enrichment, a water-soluble iron such as iron lactate, ferrous gluconate and iron sodium cirtrate, or water-insoluble iron such as ferric pyrophosphate is used. However, since water-insoluble iron has a strong iron taste, which is sensuously problematic, there has been a problem that a large quantity could not be used at a time. In addition, since the iron ionized due to the water-solubility is highly reactive to the stomach wall, there has been a problem that a water-soluble iron may be causative of an ulcer and the like in the case where a water-soluble iron is excessively taken into the body. Also, since a water-soluble iron is highly reactive to the other components in a beverage, there has been a problem that precipitation, aggregation, coloration and the like of the reactants occur. Also, in the case of water-insoluble irons, although a problem of the iron taste is overcome, still there has been a problem that it was not preferable in terms of nice external appearance as food products and the bioabsorbability was poor because the specific gravity was as high as 2.75 or more, so that the iron was precipitated in a short period of time when added and dispersed in a beverage.
Magnesium is present in the bones, muscles or other soft tissues in a living body, and about 60% is said to be present in the bones. Magnesium has actions to modulate enzymes, produce energy, regulate protein synthesis and other actions, and it is indicated that serious symptomatic changes in the organs may come out due to deficiency in magnesium intake. A food additive which can be added to foods to enrich magnesium includes inorganic salts such as magnesium chloride and magnesium sulfate, however, these inorganic magnesium salts give a bitter taste, and present a problem that aggregation or precipitation occurs during the manufacturing of processed foods.
Also, zinc is found to activate 20 kinds or more of enzymes including alkaline phosphatases, alcohol dehydroganases and the like, or to be involved in protein synthesis, nucleic acid metabolism, insulin synthesis and the like. In addition, it is known that alimentary deficiency in zinc leads to a disorder in the human body.
As described above, minerals, particularly calcium, iron, magnesium and zinc as mentioned above, have an important role in health maintenance. As such, these minerals are desirably taken in daily diet, but cannot be sufficiently taken under the current diet life of men of today.
In view of this, an attempt was made to develop mineral-enriched foods. However, mineral supplements used for manufacturing such foods have many problems on the basis of the fact that a characteristic property, that is, bioavailability (ratio of the amount absorbed in the living body to the intake amount) of minerals is low, or on the basis of the properties to react with other components or to precipitate in the food.
In the case where a water-soluble mineral is added to foods as a mineral supplement, for example, the flavors of the mineral-containing foods are significantly destroyed by salty taste, bitter taste, astringent taste and the like ascribed to mineral ions produced in the solution, in particular the anions. In addition, there are problems, including a problem that the mineral ions may react with proteins to produce a coagulated matter, and may have an adverse effect on the organization or physical properties of the food by reacting with an ingredient in the food.
Also, when a water-insoluble mineral is added, there is a problem that it is difficult to attain homogenization in foods, for example, precipitation and separation occurs in a short period time due to high specific gravity, or secondary aggregation of powder of the mineral occurs.
Since the water-insoluble mineral generally has a high specific gravity (usually 1.5 or more), the water-insoluble mineral is easily precipitated. Therefore, in order to stably disperse the water-insoluble mineral in water, it is necessitated that the water-insoluble mineral is firstly made into fine particles. In physical crushing methods using a ball mill, jet mill or the like (JP-A-Sho-57-110167), there is a limitation in obtaining fine particles having a particle size in the order of several microns, so that sufficiently stable dispersion is not obtained. As to methods for preparing fine particles having particle sizes in the order of submicrons, there have been numerously reported chemical production methods utilizing reactions for forming salts by neutralization. According to this method, there can be produced ultrafine particles having a particle size of 1/100 microns. However, the resulting ultrafine particles are immediately formed into secondary aggregates after its production, so that there poses a problem in forming coarse particles having particle sizes in the order of microns.
As a method for suppressing the formation of such coarse particles, there have been proposed methods for adsorbing and retaining primary fine particles in the network structure of polymer by adding a mucopolysaccharide such as a crystalline cellulose, pectin, carageenan and guar gum (JP-A-Sho-56-117753, JP-B-Sho-57-35945 and JP-A-Hei-09-191855); methods of alleviating specific gravity by adding a water-insoluble mineral to fats and oils and dispersing the water-insoluble mineral therein, adjusting the content of the fats and oils in the resulting mixture so as to have the content to be 30% by weight or more (JP-A-Sho-57-110167); and the like. In any of these methods, however, since it is needed to add large amounts of the substances other than the targeted water-insoluble mineral, there are defects in that the solute dispersed therein is diluted, and that the dispersibility of the water-insoluble mineral is drastically lowered. As to methods for eliminating the above defects, there have been developed a method for treating surfaces of fine particles of a water-insoluble mineral with an organic acid or an alkaline agent (JP-A-Sho-61-15645); a method of treating with a surfactant, such as sucrose ester (JP-A-Sho-63-173556 and JP-A-Hei-5-319817), and the like. In the former method, however, there is a defect in that the metal ions and the like constituting the water-insoluble mineral are likely to be made into free ions in the water phase. In addition, in the latter method, there are defects in that the surfactant layer formed by adsorption to the surfaces of the fine particles is peeled off when subjected to a heat treatment such as sterilization, and that secondary aggregation is accelerated.
Also, there have been proposed a process for preparing a calcium carbonate dispersion by formulating a hydrophilic emulsifier having an HLB of 10 or more to an aqueous calcium suspension, and grinding the mixture using a wet grinder (JP-A-Hei-06-127939), a process for preparing a calcium carbonate dispersion by formulating a hydrophilic emulsifier having an HLB of 10 or more to an aqueous calcium phosphate suspension, and grinding the mixture using a wet grinder (JP-A-Hei-06-127909), and the like. The main purpose of these processes is to prevent secondary aggregation of powder. Although these methods improve the dispersibility upon addition to food, the effect to retain the dispersibility for a long period of time is insufficient in foods to which the dispersion is added, in particular liquid foods. In addition, the hydrophilicity of the emulsifier to be added is high, and a large amount of bubbles are formed when a homogenizer is used or agitation is performed after the emulsifier is dissolved in an aqueous phase, which was problematic to the subsequent operation. In addition, for a highly hydrophilic emulsifier, even when a layer of the emulsifier is once formed by adsorption to the surface of the particles, the emulsifier is easily dissolved in the water solvent by monodispersion or micelle formation, so that secondary aggregation could not be completely prevented.
Further, with respect to the bioabsorbability of mineral, not all the minerals mentioned above are completely absorbed and the bioavailability is in the order of about a few percent to a few tens percent, there has been a problem that most of the minerals are excreted without being available in the living body. Due to the above-mentioned dispersing techniques of formulating a mucopolysaccharide, a hydrophilic emulsifier or the like to finely grained mineral, there is found an effect of improvement in dispersion stability to some extent, but not completely, as compared with finely grained mineral not formulated with a mucopolysaccharide or a hydrophilic emulsifier. However, after being added to mineral-enriched food and taken into the body, an improvement in bioabsorbability, that is bioavailability, could not be expected considering the properties of the above mucopolysaccharide and the like to be formulated. Because a mucopolysaccharide prevents secondary aggregation by supporting finely grained mineral in its three-dimensional network structure which provides steric hindrance but does not have any effect on the absorption of finely grained mineral. Also, the hydrophilic emulsifier prevents the secondary aggregation by forming a coating on the surface of finely grained minerals. However, when added to a mineral enriched food, since the material constituting the coating is hydrophilic, the coating is easily peeled off and transferred to the aqueous phase. Hydrophilic polyglycerol fatty acid esters, sugar esters and the like have a large molecular weight, and a low biocompatibility due to a large difference in polarity therebetween, as compared with molecules constituting the biomembrane, such as phospholipids. On the other hand, in minerals prepared by coating with phospholipids such as lethitin, since the electric charges from the functional groups of the phospholipids tend to form a salt together with the charges possessed by the mineral, coarse mass of the prepared particles due to secondary aggregation is likely to be produced, so that it was difficult to finely grain and formulate the particles. In addition, there has been proposed a method for improving dispersion stability by formulating an enzymatically decomposed lecithin with a mineral composition (Japanese Patent No. 3050921). In this method, it is difficult to control the fineness of particles due to the influence of the charges from the functional groups of phospholipids. For a water-insoluble mineral prepared by formulation an enzymatically decomposed lecithin during the special reaction for forming salt by neutralization, the dispersion stability is relatively improved. However, since a highly hydrophilic nonionic surfactant is formulated as an emulsifier coating, the coating is peeled off and transferred to the aqueous phase due to the dilution of the emulsifier upon addition to food. Therefore, it could not be sufficiently expected that the absorbability in the body in the intestinal tract was improved.